1. Field of the Invention
The present invention relates to a system for monitoring the conditions of a fine wire bonded interconnection and more particularly to a monitoring system which detects the presence and length of a wire tail employed to make a ball in preparation for making proper wire bonded interconnections.
2. Description of the Prior Art
Automatic wire bonders that employ computer controls are known and described in U.S. Pat. Nos. 4,266,710 and 4,239,144 assigned to Kulicke and Soffa Industries, Inc., the same assignee as the present invention. Such high speed automatic wire bonders are capable of making eight complete wire interconnections per second. If the fine wire used to make these interconnections breaks improperly and/or is not properly fed through the bonding tool, a tail of proper length is not positioned below the working face of the bonding tool at the time a ball is made in preparation for the next interconnection. Both wedge bonders and ball bonders require a tail of proper length in order to continue to make proper bonds. Numerous problems occur which can cause a fine wire to be missing, too short or too long which will effect the ball size which is produced by an electronic flame off (EFO) device.
Very large scale integration (VLSI) devices are becoming more dense and are employing a greater number of lead out pads (electrodes) that must be wire bonded to lead connections on carriers or packages. As a result of the more dense devices, the lead out pads are becoming smaller and more fragile and the VLSI devices are becoming more expensive. When an automatic wire bonder attempts to bond a fine wire to a lead out pad and there is no fine wire, or no ball under the working face of the bonding tool, the bonding tool makes direct contact with the lead out pad and can easily destroy the pad and the very expensive VLSI device on the attempt to make the next wire bond interconnection.
Accordingly, it is becoming increasingly important to provide an apparatus and a method of monitoring the length and presence of a bonding tail that is made by an automatic wire bonder after the second bond of a fine wire interconnection to prevent the bonding tool from attempting to make a first or ball bond without a proper size ball under the working face of the bonding tool.